The present invention relates generally to travelling wave tubes and, more particularly, to an improved collector for use in such tubes. As is known, a travelling wave tube comprises a wave propagating structure including an elongated tube in which an injected stream of electrons interacts continuously with the field of a guided electromagnetic wave moving substantially in synchronism with the stream of electrons such that there is a net transfer of energy from the stream to the wave.
Travelling wave tubes are commonly used in the communications field for amplification of radio-frequency signals. One type of travelling wave tube is disclosed in Reference Data For Radio Engineers, Pages 17-20 and 17-21. In such devices, the electron beam produced by an electron gun is directed through the elongated tube and is collected at the opposite end of the tube by a collector electrode. Usually the collector is in the form of a metallic central element that is surrounded by a ceramic insulator which has an inner metallized surface so that the electrode can be secured to the surface thereof.
One of the problems encountered with traveling wave tubes is the dissipation of the heat that is developed in the collectors, particularly multi-stage collectors that produce relatively high power outputs. Because of the differences in thermal expansion coefficients between the metal electrode, which is usually copper, and the ceramic insulator, significant tensile hoop stresses will be developed in the ceramic. Also, severe tensile stresses will arise between the metal and ceramic surfaces of the collector which are brazed to each other. Because of these high thermally-induced stresses, the ceramic insulator may fracture after only short periods of temperature cycling. In other instances, the metallized surface of the ceramic insulator may be pulled from the insulator by the metal.
Various techniques have been developed by various manufacturers to keep the thermal stresses within acceptable limits. One type of air-cooled collector that has been proposed has a plurality of elongated slots that are directed inwardly from the outer peripheral surface of the metal electrode to allow the metal electrode to expand without fracturing the ceramic insulator around the electrode. However, it was determined that such an arrangement resulted in a very high stress concentration at the joint between one edge of the slot and the adjacent metallized surface of the ceramic insulator. Analysis has indicated that only a slight increase in yield strength of the metal electrode would drive the stress in the braze joint above the tensile strength of the metallizing on the surface of the ceramic and, as stated previously, would result in failure due to the separation of the metallized surface from the ceramic insulator.